We present a compact detector module for gamma detection in the PET part of a simultaneous ToF-PET/MR system. The module covers an area of 3.3cm x 3.3cm with 64 SiPM based readout channels. It is composed of a stack of three PCBs of identical size: The SiPMs on the topmost PCB are read out by two full-custom ASICs located on a second PCB located underneath. A third PCB at the bottom of the stack contains a local voltage regulator, an FPGA for ASIC control and data processing, and DACs to generate bias voltages for the readout ASICs and the SiPM devices. An LYSO scintillator block is optically coupled to the SiPMs for gamma to light conversion.